The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.
(1) Field of the Invention
The present invention relates generally to communications antennas, and more particularly to plasma antennas adaptable for use in any of a wide range of frequencies
(2) Description of the Prior Art
A specific antenna typically is designed to operate over a narrow band of frequencies. However, the underlying antenna configuration or design may be adapted or scaled for widely divergent frequencies. For example, a simple dipole antenna design may be scaled to operate at frequencies from the 3-4 MHz band up to the 100 MHz band and beyond.
At lower frequencies the options for antennas become fewer because the wavelengths become very long. Yet there is a significant interest in providing antennas for such lower frequencies including the Extremely Low Frequency (ELF) band, that is less than 3 kHz, the Very Low Frequency (VLF) band including signals from 20 kHz to 60 kHz and the Low Frequency (LF) band with frequencies in the 90 to 100 kHz band. However, conventional half-wave and quarter-wave antenna designs are difficult to implement because at 100 Hz, for example, a quarter-wave length is of the order of 750 km.
Notwithstanding these difficulties, antennas for such frequencies are important because they are useful in specific applications, such as effective communications with a submerged submarine. For such applications, conventional ELF antennas comprise extremely long, horizontal wires extended over large land areas. Such antennas are expensive to construct and practically impossible to relocate at will. An alternative experimental Vertical Electric Dipole (VEP) antenna uses a balloon to raise one end of a wire into the atmosphere to a height of up to 12 km or more. Such an antenna can be relocated.
To be truly effective the antenna should extend along a straight line. Winds, however, can deflect both the balloon and wire to produce a catenary form that degrades antenna performance. Other efforts have been directed to the development of a corona mode antenna. This antenna utilizes the corona discharges of a long wire to radiate ELF signals.
Still other current communication methods for such submarine and other underwater environments include the use of mast mounted antennas, towed buoys and towed submersed arrays. While each of these methods has merits, each presents problems for use in an underwater environment. The mast of current underwater vehicles performs numerous sensing and optical functions. Mast mounted antenna systems occupy valuable space on the mast which could be used for other purposes. Consequently, as a practical matter, the use of such antennas for ELF or other low frequency communications is not possible because they require too much space. For both towed buoys and towed submersed arrays, speed must be decreased to operate the equipment.
Conventional plasma antennas are of interest for communications with underwater vessels since the frequency, pattern and magnitude of the radiated signals are proportional to the rate at which the ions and electrons are displaced. The displacement and hence the radiated signal can be controlled by a number of factors including plasma density, tube geometry, gas type, current distribution, applied magnetic field and applied current. This allows the antenna to be physically small, in comparison with traditional antennas. Studies have been performed for characterizing electromagnetic wave propagation in plasmas. Therefore, the basic concepts, albeit for significantly different applications, have been investigated.
With respect to plasma antennas, U.S. Pat. No. 1,309,031 to Hettinger discloses an aerial conductor for wireless signaling and other purposes. The antenna produces, by various means, a volume of ionized atmosphere along a long beam axis to render the surrounding atmosphere more conductive than the more remote portions of the atmosphere. A signal generating circuit produces an output through a discharge or equivalent process that is distributed over the conductor that the ionized beam defines and that radiates therefrom.
U.S. Pat. No. 3,404,403 to Vellase et al. uses a high power laser for producing the laser beam. Controls repeatedly i pulse and focus the laser at different points thereby to ionize a column of air. Like the Hettinger patent, a signal is coupled onto the ionized beam.
U.S. Pat. No. 3,719,829 to Vaill discloses an antenna constructed with a laser source that establishes an ionized column. Improved ionization is provided by means of an auxiliary source that produces a high voltage field to increase the initial ionization to a high level to form a more highly conductive path over which useful amounts of electrical energy can be conducted for the transmission of intelligence or power. In the Hettinger, Vellase et al. and Vaill patents, the ionized columns merely form vertical conductive paths for a signal being transmitted onto the path for radiation from that path.
U.S. Pat. No. 3,914,766 to Moore discloses a pulsating plasma antenna, which has a cylindrical plasma column and a pair of field exciter members parallel to the column. The location and shape of the exciters, combined with the cylindrical configuration and natural resonant frequency of the plasma column, enhance the natural resonant frequency of the plasma column, enhance the energy transfer and stabilize the motion of the plasma so as to prevent unwanted oscillations and unwanted plasma waves from destroying the plasma confinement.
U.S. Pat. No. 5,594,456 to Norris et al. discloses an antenna device for transmitting a short pulse duration signal of predetermined radio frequency. The antenna device includes a gas filled tube, a voltage source for developing an electrically conductive path along a length of the tube which corresponds to a resonant wavelength multiple of the predetermined radio frequency and a signal transmission source coupled to the tube which supplies the radio frequency signal. The antenna transmits the short pulse duration signal in a manner that eliminates a trailing antenna resonance signal. However, as with the Moore antenna, the band of frequencies at which the antenna operates is limited since the tube length is a function of the radiated signal.
Notwithstanding the disclosures in the foregoing references, applications for ELF frequencies still use conventional land-based antennas. There remains a requirement for an antenna that can be mast mounted or otherwise use significantly less space than the existing conventional land-based antennas for enabling the transmission of signals at various frequencies, included ELF and other low-frequency signals, for transmission in an underwater environment.
Accordingly it is an object of the present invention to provide an antenna capable of operation with ELF signals.
Another object of this invention is to provide an antenna that is capable of transmitting signals in different frequency ranges including the ELF range.
Still another object of this invention is to provide an ELF antenna that is transportable.
Yet another object of this invention is to provide an ELF antenna that can be mounted in a restricted volume.
In accordance with this invention, an antenna for operating at a reference frequency radiates a field by repetitively producing a plasma in a confined, vertically extending volume. The plasma has a characteristic relaxation time when the ionizing process ceases. The interval between successive repetitive energizations is less than the characteristic relaxation time.
In accordance with another aspect of this invention, a communications system for operating at a reference frequency includes a high-power laser that generates a laser beam along an axis positioned to be vertically directed into the atmosphere. The laser operates in a pulsed manner to produce a vertical plasma column in the atmosphere. The plasma has a characteristic relaxation time, and the interval between successive pulses applied to the laser is less than the characteristic relaxation time. A modulation signal controls the pattern of the repetitive pulsing and operates at a reference frequency that is less than the pulse repetition frequency for energizing the laser. Alternately exciting and extinguishing the plasma in response to the modulating signal enables a current having alternating directions to be developed in the plasma at the reference frequency.